Guide to the night: Sarah Hall goes swimming in the night

One autumn night a few years ago, I found myself at the Outer Banks of North Carolina – that impossibly thin spit of land running along the state's coastline, from which the Wright brothers made the first momentary flight. It was late into hurricane season, the daylight had gone west, and the ocean was relatively calm. Standing on the narrow sands in the salty gloom, I could just make out the breakers rolling in.

I was supposed to be at a wedding party, but the beach was empty, I was in the process of getting divorced, and the sea was performing an act of hypnosis. Perhaps going in was a solo trial of some kind, a facing down of metaphysical fears, or perhaps the nocturnal scene was just too affecting, too intoxicating. Either way, the circumstances were perfect, and I knew I was going to do it. No riptide warnings had been issued, and the anecdotes I'd heard about sharks preferring to take their meals in the evening seemed unlikely. I left my party dress in a heap on the dunes.

Getting into the ocean was initially an exercise in finding the right set of waves to go under or over, and, because visibility was poor, I had to rely on my hearing. This one sounds too big. This one is already cresting. It was terrifying. There's nothing like the vast, dark Atlantic to remind you of your mortality. But terror can also be exhilarating.

Somehow I managed not to get tumbled in the surf. I didn't swim far out: I wasn't suicidal – I just wanted to be held and rocked, or to register my choices perhaps. The blind, rollercoaster swell was giddying and unnerving, the brine brisk and zesty. And I was fine, buoyant, waterproof: a creature suited to its environment. Then, as if to confirm this natural order, the most lovely of occurrences: a bloom of phosphorescent plankton came in on the tide. Below me, the water sparked and glittered. Above, the luminosity repeated.

Such experiences are rare, and such activities are hazardous. No doubt there's something about being British and abroad that encourages eccentric behaviour. But what of our domestic, moonlit waterways? Might they too not provide us with adventure and reward in these last days before the onset of polar weather? If, at the very mention of such a thing, frost has gripped your bones and you're tempted to turn the page, bear with me. Consider the temperature of the seas around our coastline, the temperature of the rivers and lakes and mountain streams. It's always fairly chilly, no matter the date on the calendar.

Swimming in the UK is not really about enjoying a sultry experience. It's about cold, clear acts of purification, and constitutional durability. It's about invigoration and bravado. In, out: it's usually all over very quickly; the limbs left trembling, the brain pitching a soprano note inside the skull. Swimming in the cold and the dark of British autumn is not for the faint-hearted.

Nor, perhaps, should it be recommended. There is much jeopardy when it comes to chucking (or easing) yourself into unseen, gelid, moving bodies of water. Heart attack. Hypothermia. Dangerous currents. Giant, man-eating pike. Injurious obstacles. Mucky floating stuff. So let's get the health and safety bit out of the way now. Don't do it. It's perilous. It's catastrophically shocking to the human thermal system. It's pore-puckering madness. But, for the peculiar few, including me, late-season swimming is a wildly satisfying prospect, and worth all the pain.

I have several friends who are members of outdoor bathing clubs. These hardy folk do it year round, including Christmas Day. They do it at dawn, dusk or in pitch blackness. I suspect their robustness of health is not unconnected. Being in a group of like-minded enthusiasts does lessen the erosion of your resolve, if you've casually tested the water with a hand and found it to be glacial. I've enjoyed toeing down ladders into the green reedy Cam alongside six-year-olds and nude octogenarians, or jumping off packhorse bridges in Wales, Devon and Northumberland with fellow daredevils. Such camaraderie is brilliant, and may we always shoal.

But I'm most fond of my local haunts. Here in the Lake District we are spoiled for water. Around every corner there's something wet and beautiful and inviting: a river, a tarn, a gill, a mere. Slate-bottomed and exotically blue, cataract white, or peat-steeped brown; our water is full of character. You can't actually see such beauty while swimming, but sometimes not seeing a thing makes it feel all the better. Night swimming is more about sensory experience than aesthetic.

My favourite pool is located in a remote valley in the eastern Lake District, surrounded by vine-hung cliffs and slippery boulders. It has a torrential sheet waterfall at one end and is almost black in colour, so it appears bottomless, a portal to nowhere. Even in daylight, it is a very scary place. Picture Death's bathtub. I have swum in it only once at night, earlier this year, and I confess it was too much for me. I got the fear.

Perhaps it was the blind rushing sound of the waterfall, or the brackish wash spiralling around my neck and shoulders, or perhaps it was the vertiginous Cumbrian night sky that sent me over the edge. In the eerie lapping pitch I suddenly panicked. I became convinced the chain had been pulled and I was about to go swirling down the universe's plughole. Never have I felt such a strong imperative to get out of the water. Sometimes challenging yourself in a dark, fearful arena will lead to defeat rather than victory.

My top tip, if you are thinking of taking the plunge, is this: consider every night swim to be a potential finale. You'll probably live through it, but you may want to pen your last wishes or a note to loved ones.

Here's another tip: if engaging in the traditional version of the sport (ie no wetsuit), it's vital to carry a towel or an item that will serve as one. I inherited a curious habit from my father – another kamikaze bather – who always carted a piece of old chamois leather around with him in case he went swimming. It was used, when not being employed to clear condensation from the car windscreen, to hastily dry him off after he'd leapt into some upland spa. Other recommended pieces of kit are as follows: blanket, flask of tea, tot of whisky, full moon or headlamp. Swimsuit optional.

The world can probably be divided into two kinds of people: those who enjoy the tonic sensation of cold water in darkness, and those for whom it is absolute torture. For the former, the recompense of such a baptism is worth every nervous, bitter moment. And even in October, while ice crystals form in the earth's mantle, the wind stokes up the fallen leaves and decay musks the air, a midnight dip might be just the thing to lift your spirits and ready you for winter.

Imagine it. You are poised in the shallows, your eyes owl-like in the gloaming, the borders between solids and liquids merging. The thin glimmering element is stinging your ankles and shins. You aren't sure whether to fully broach the surface or not, until a curious logic occurs to you. You're already immersed in the cool dark density of night, so immersing yourself in a thicker, darker coldness makes sense. You take the next step, and the next, yield to the arctic burn on your thighs, waist, your tender neck. At first the sensation is electric, almost unbearable, yet bearable. Lung and nerve and blood mechanisms go into shock. Your body enters an elation of rage, because an extreme thing is happening. An adrenaline supernova follows, a burst of emergency energy. After a second or two your system recalculates, adjusts; there is a brief physiological acceptance.

And then you are swimming. There may only be a minute's worth of swimming, and it may resemble a desperate dog paddle rather than an Esther Williams performance – doesn't matter, there are no witnesses – but that minute is a rare, certain period in life. You are extraordinarily alive during it. You body is assuring you that it wants to live. If you can stand to submerge the crown of your head, the thing to do is turn on your back and look up at the stars. You will suddenly understand their symmetry. You too are alone and floating in the cold beauty of space.

Once you're out, and have dressed cosily in dry layers, and the whisky is burning its way through your core, how marvellous the world is, how refreshed and radiant. You've suffered. You've survived. You are cleansed. Though it is almost too dark to see, you are, in a way, enlightened.

• Sarah Hall's latest novel, How to Paint a Dead Man, is published by Faber and Faber